The present invention concerns a voltage controlled oscillator used in local oscillators and symthesisers of radio equipments for microwave and millimeter-wave frequency systems.
The present invention is used in a personal radio tranceiver such as a gate card or a security card, an AWA (ATM wireless access) radio terminal, and a mobile terminal, in which an oscillator is essential.
A local oscillator is an essential component in a micro-wave transmitter/receiver equipment. There is a tendency to provide a local oscillator in a monolithic micro-wave integrated circuit (MMIC) so that a small sized, low-cost, and high function radio tranceiver is obtained.
A conventional PLL (phase lock loop) local oscillator has high frequency stability and low phase noise close to a carrier frequency because of the use of a crystal oscillator as reference signals. A PLL local oscillator comprises essentially a voltage controlled oscillator, a frequency divider, and a phase frequency comparator. Among them, a voltage controlled oscillator is a key component for defining characteristics of a PLL local oscillator, and therefore, it is desired that it has high frequency stability, high output level, wide frequency vary range, and low phase noise. In particular, a software radio (for instance, a Personal Digital Cellular (PDC) phone in Japan, Global System for Mobile communications (GSM) in Europe, and IS-95 in U.S.A. are handled by a single tranceiver by switching with a software) has been studied so that various frequency bands, various modulation schemes, and various data formats are handled by modifying a software. Therefore, an oscillator which has wide frequency tuning range has been strongly required so that it meets with requests for various frequency bands.
FIG. 25 shows a prior voltage controlled oscillator (I. D. Robertson, "MMIC Design", The Institution of Electrical Engineers, London, UK, p.364,1995, ISBN 0-85296-816-7). In the figure, the numeral 100 is an FET which operates as an oscillation element with a gate terminal 111 grounded through an inductor 121 and coupled with an anode 151 of a varactor 150 which is a voltage controlled variable capacitance element through an inductor 120. A cathode 152 of a varactor diode 150 is coupled with a control voltage input terminal 114 through a resistor 140, and is grounded through a capacitor 130. A source terminal 113 is coupled with an inductor 122 and a capacitor 131. The inductor 122 is grounded through a resistor 141, and the capacitor 131 is directly grounded. A drain terminal 112 is coupled with a drain voltage supply terminal 115 through an inductor 123, and also coupled with an output terminal 110 through an inductor 124 and a capacitor 133. The drain voltage supply terminal 115 is grounded through a capacitor 132. An output matching circuit is composed of inductors 123 and 124, and capacitors 115 and 133.
When said terminal 115 is supplied with drain voltage, the gate of the FET 100 provides capacitive negative impedance, and therefore, a voltage controlled oscillator is obtained by coupling a variable inductance with the gate 111 of the FET. The variable inductance in the prior art is composed of a varactor diode 150 and an inductor 120. Said varactor diode 150 is coupled with a source and a drain of the FET, and controls parallel sum of capacitance C.sub.gs between a gate and a source, and capacitance C.sub.gd between a gate and a drain according to the control voltage. The circuit of FIG. 25 may be integrated on a semiconductor substrate, and provides a reproducible and a low cost circuit.
However, the circuit of FIG. 25 which has a varactor diode for providing a variable reactance has the disadvantage that the control range of the capacitance C.sub.gs between a gate and a source is only around 1:2 or less when it is implemented as a monolithic micro-wave integrated circuit (MMIC) integrated on a semiconductor substrate, and resultant frequency control range is also small.
Another disadvantage of the circuit of FIG. 25 is that it needs more than two active elements, a negative impedance element (FET) and a varactor. This means that the yield rate in production decreases in higher operational frequency. Further, phase noise characteristics is degrades, since low frequency noise component (1/f noise) is up-converted up to oscillation frequency through mixing effect by non-linear characteristics by a plurality of active elements.
FIG. 26 shows another prior voltage controlled oscillator which is shown in JP patent laid open 140836/1994. In the figure, the numeral 200 is a transistor which operates as an oscillation element, with a base terminal 211 grounded through a resistor 208 and coupled through a capacitor 201 with a transmission line 210, the other end of which is grounded through a capacitor 202. Said base terminal 211 is further coupled with a base voltage control terminal 214 and a capacitor 203 through a resistor 207. The other end of the capacitor 203 is grounded. The emitter terminal 213 of the transistor 200 is grounded through a parallel circuit of a resistor 209 and a capacitor 204, and is further coupled with the base terminal 211 through a capacitor 205. The collector terminal 212 of the transistor 200 is connected to a collector voltage control terminal 215, which is grounded through a capacitor 206.
In the circuit of FIG. 26, the resistor 209 provides negative resistance so that the base terminal 211 of the transistor 200 provides capacitive negative impedance. Therefore, when an inductive transmission line 210 is coupled with the base terminal, the circuit of FIG. 26 works as an oscillator. The oscillation frequency of the voltage controlled oscillator depends upon the voltage difference between the base voltage control terminal 214 and the collector voltage control terminal 215 so that the capacitance between the base and the collector is adjusted.
The circuit of FIG. 26 has the advantage as compared with the circuit of FIG. 25 that no varactor diode which is a variable reactance element is necessary, but only one transistor is enough as an active element for providing a frequency controlled oscillator. Therefore, the circuit of FIG. 26 improves the producing yield rate approximate twice as compared with the circuit of FIG. 25, and further decreases non-linear element in a feedback circuit so that phase noise is lowered.
However, the circuit of FIG. 26 has the disadvantage that the control range of the capacitance between the base and the collector is less than 1:2, therefore, large control range of the oscillation frequency is impossible in the circuit of FIG. 26.
The circuit of FIG. 26 is further analyzed for D.C. (direct current) operation. The current in a transistor depends upon the voltage between a base and an emitter. In the case of npn transistor, when voltage is applied to a base, potential barrier of a base-emitter junction is decreased, an electron in an emitter is injected into a base, and if width of a base is sufficiently small, almost all the electrons injected to a base reach a base-collector junction and flows into a collector (current flows from a collector to an emitter). Therefore, voltage between a base and an emitter must be controlled freely so that base current (or emitter current) of a transistor is adjusted.
However, the resistor 209 which operates to provide negative resistance in the voltage controlled oscillator in FIG. 26 restricts the range of the voltage between a base and an emitter. The voltage V.sub.ee applied to the emitter terminal 213 must shift by voltage drop by emitter current I.sub.e in the resistor 209. Even if voltage at the base voltage control terminal 214 is increased so that base current (or emitter current) increases, the voltage between the base and the emitter does not so increase because of the voltage drop in the resistor 209, and therefore, the base current does not increase. To confirm above principle, the considerable circuit model using commercial transistors are calculated. The calculated results is explained in detail in accordance with FIG. 27.
It is assumed that voltage is applied to a base through a resistor of 1 K.OMEGA., and to a collector through an inductor of 1 .mu.H, and collector voltage is fixed to 1 V. A transistor used for calculation is an SSTIC bipolar transistor (C. Yamaguchi, Y. Kobayashi, M. Miyake, K. Ishii, and H. Ichino, "0.5-.mu.m bipolar Transistor Using a New Base Formation Method; SSTIC," in IEEE 1993 Bipolar Circuits and Technology Meeting, 4.2, pp.63-66). The size of an emitter is 0.3 .mu.m.times.120.6 .mu.m. The current-voltage characteristic of a transistor itself is shown in FIG. 28.
An SSTIC bipolar transistor increases exponentially emitter current and base current as the base-emitter voltage increases as is the case of an ordinary transistor.
FIG. 29 shows numerical calculation showing relationship between voltage of a base voltage control terminal, and voltage between a base and an emitter, in the circuit of FIG. 26, where R.sub.1 is resistance of the resistor 209. It should be noted that as the resistance coupled with an emitter increases, the maximum voltage between a base and an emitter decreases, and therefore, base current (and emitter current) flowing in a transistor decreases.
Therefore, the circuit of FIG. 26 can not provide large emitter (or collector) current, and it is difficult to provide large oscillation output. Further, in the circuit of FIG. 26, the resistance of the resistor 209 must be high to provide large negative resistance, and the resistor 209 is essential for oscillation operation. If the resistance of the resistor 209 is zero, no negative resistance is provided, and no oscillation operation is provided.
As described above, the circuit of FIG. 26 has the disadvantage that the control range of the voltage between a base and an emitter is restricted, although the voltage between a collector and a base is controllable arbitrary.